A shape of the Brillouin gain spectrum (which will be referred to hereinafter as a “BGS”) which is a spectrum of a gain by the Brillouin scattering generated in an optical fiber changes with temperature and distortion of the optical fiber. A technology which measures the temperature and distortion using the change of shape of BGS is known.
As the technology which measures BGS, BOCDA (Brillouin Optical Correlation Domain Analysis) is known for example, (see Non-Patent Document 1). In the BOCDA, pumping light and probe light are made to be inputted opposing each other into both ends of the optical fiber, and at the same time, sweeping a difference (which will be referred to hereinafter as an “optical frequency difference”) of each center frequency of the probe light and the pumping light, BGS of the gain generated at the position where the phase coincides with each other is measured.    Non-Patent Document 1: Kazuo HOTATE, et al., “Simplified System of Fiber Brillouin Optical Correlation Domain Analysis for Distributed Strain Sensing”, International congress on 16th optical fiber sensors (OFS-16), We2-3, p. 290-293, October 2003.